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Ch.17 - Additional Aspects of Aqueous Equilibria
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.17 - Additional Aspects of Aqueous EquilibriaProblema 86
Capitolo 17, Problema 86

The acid–base indicator bromcresol green is a weak acid. The yellow acid and blue base forms of the indicator are present in equal concentrations in a solution when the pH is 4.68. What is the pKa for bromcresol green?

Guida verificata passo dopo passo
1
Identify that the problem involves an acid-base indicator, which is a weak acid, and its pKa value needs to be determined.
Recall that when the concentrations of the acid form and the base form of an indicator are equal, the pH of the solution is equal to the pKa of the indicator.
Understand that the given pH value of 4.68 is the point at which the concentrations of the yellow acid form and the blue base form of bromcresol green are equal.
Conclude that the pKa of bromcresol green is equal to the pH at which the concentrations of the two forms are equal, which is 4.68.
Therefore, the pKa for bromcresol green is 4.68.

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Acid-Base Indicators

Acid-base indicators are substances that change color in response to changes in pH. They are typically weak acids or bases that exhibit different colors in their protonated and deprotonated forms. For example, bromcresol green appears yellow in acidic conditions and blue in basic conditions, making it useful for determining the pH of a solution.
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Acid-Base Indicators

pKa and pH Relationship

The pKa of an acid is the negative logarithm of its acid dissociation constant (Ka) and represents the pH at which the concentrations of the protonated and deprotonated forms of the acid are equal. When the pH of a solution equals the pKa, the acid and its conjugate base are present in equal amounts, which is crucial for understanding the behavior of weak acids and bases in solution.
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pH and pOH Calculations

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a solution to the pKa of an acid and the ratio of the concentrations of its deprotonated and protonated forms. It is expressed as pH = pKa + log([A-]/[HA]). This equation is particularly useful for calculating the pKa when the pH and concentrations of the acid and base forms are known, as in the case of bromcresol green.
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Henderson-Hasselbalch Equation
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